Patent Documents 1 and 2, for example, propose the switching power supply devices that are designed to improve the power factors thereof by performing the PFC (Power Factor Correction).
In order to reduce switching loss and conduction noise, each of these switching power supply devices according to Patent Documents 1 and 2 lowers the switching frequency around the peaks and bottoms (the peaks, in case of a fully rectified waveform) of an input AC power supply voltage.
In an ordinary switching power supply device, a switching frequency is set within the range of 50 kHz to 70 kHz.    Patent Document 1: Japanese Patent Application Publication No. 2010-233368    Patent Document 2: Japanese Patent Application Publication No. 2010-273420
The control methods described in Patent Documents 1 and 2 cannot achieve both the reduction of EMI (Electro Magnet Interference: electromagnetic noise) and the improvement of the conversion efficiency.
In other words, the bottleneck in reducing EMI is normal noise, a type of conduction noise. Because the standard conduction noise frequency is 150 kHz to 30 MHz, the basic component and second harmonic component thereof is not perfect for a 50 to 70 kHz switching frequency. For this reason, the maximum value of the normal noise is determined based on a third harmonic component of ripples of current flowing to an inductor. Unfortunately, the control methods described in Patent Documents 1 and 2 are not adaptable to third harmonic components and therefore cannot achieve the reduction of EMI and the improvement of conversion efficiency.